1. Field of the Invention
The present invention relates to instruments. More precisely, the present invention relates to a digital force balanced instrument that incorporates a directly digital electrostatic forcer with servo loop plates charged by a digital feedback loop. The present invention is related to co-pending U.S. patent application Ser. No. 08/097,084, filed Jul. 26, 1993, entitled Electrostatically Force Balanced Silicon Accelerometer.
2. Prior Art and Related Information
Precise instruments such as accelerometers are used in a variety of applications, such as in measuring the acceleration or deceleration of a ship, missile, or airplane. A conventional accelerometer uses a pendulum type transducer in which acceleration is detected by noting the displacement of the pendulum. A force is applied to the pendulum, generally by an electromagnetic field, in order to force the pendulum back to its initial, pre-displacement position. By detecting the current required to generate the electromagnetic field, one could determine the acceleration. A more recent accelerometer design relies on a movable, silicon electrode that is positioned between two fixed electrodes while cantilevered to a silicon base. The movable electrode is spaced apart from two fixed electrodes. The entire structure is sandwiched within a glass shell and electrically connected to monitoring circuitry.
Within any system using inertial instruments such as accelerometers, there is a computer that operates on the data provided by the inertial sensor or accelerometer. It is highly desirable for the inertial instruments to output digital data directly to the computer, thereby eliminating the need for complicated quantizer circuitry. Indeed, it is much easier to transmit and operate on a digital representation of a signal rather than on the older, analog form.
There have been attempts to build a digital output accelerometer using electrostatic forcing. These generally fall into two classes: charge controlled (see U.S. Pat. No. 5,142,921), and voltage controlled (see U.S. Ser. Nos. 07/514,114, now abandoned, and 07/908,520, now U.S. Pat. No. 5,277,053). The present invention instrument is voltage controlled, meaning that voltages are applied to the electrostatic forcing plates to produce a force. Also related is Ser. No. 08/097,084 entitled Electrostatically Force Balanced Silicon Accelerometer disclosing processes for fabricating electrostatic forcers.
Also, U.S. Pat. No. 5,142,921 to Stewart et al. discloses a force balanced instrument with electrostatic charge control. More precisely, the invention is directed to a force balanced instrument employing a pendulous mass having combined electrostatic pickoff and forcing plates on opposite sides thereof. The operative principle in the instrument dictates that the resulting force is proportional to the square of the applied voltage.
This principle, known as the square law, necessitates use of a microprocessor to take the square root of the voltage in order to obtain the force. This is a relatively complicated and expensive method, and is inherently non-linear. Potential error sources include numeric round-off in the square root routine or look-up tables of any software used. Also, any rounding or time-constant associated with the application of a voltage pulse exhibits different force weight with different pulse heights.
Another method of linear force balance is achieved by controlling the duty cycle so that the difference in duration between each of the parts of a full cycle is a linear measurement of acceleration. One drawback to this approach is that it tends to use maximum allowable voltage at all times, such that there is a tendency to suddenly move the proofmass in opposite directions. This is commonly referred to as "pinging."
For good bias stability, the application of a large force to the proofmass is to be avoided when only a small force is needed to return the proofmass to the null position. With a large force applied to the proofmass, the electrostatic negative spring rate problems increase, and temperature-dependant non-linearities become worse. One approach to overcoming such pinging problems is to use a high speed clock for fine resolution; however, for many applications, that approach causes an unacceptable increase in the power demand.
Previous voltage-biased, linearized force methods exhibit very large negative spring rates as a result of the electrostatic field from the bias voltage which is always present. The method described in U.S. patent application Ser. No. 07/514,114 does not employ a linearizing bias field, but applies a voltage to plates on one side of the accelerometer proofmass or the other depending on which direction the force is applied.
Accordingly, a need presently exists for a digital force balanced instrument that provides an output digital code that is directly linear with input force. This would eliminate the square law problems.